Power lift gates and power doors are popular on automotive vehicles such as mini-vans, station wagons, and SUVs. Many of these power lift gates and doors are powered by a DC electric motor placed in an H-bridge driver to supply the electric motor with pulsed power which may be modulated to control the speed of the motor. This controlled pulsing of the power is commonly referred to a pulse width modulation (PWM) and can be applied to power the motor in either the forward or reverse direction through the H-bridge driver.
The PWM control of the DC motor controls the speed of the motor by modulating the length of the individual pulses of power sent to the motor to derive an average power. For automotive application, the PWM frequency is confined to the low frequency spectrum from about 50 HZ to only a few hundred HZ to reduce any high frequency electromagnetic interference. The PWM cycle is divided into two portions, one portion is the power pulse period which powers the motor and the second portion is an open circuit when no power is applied. The average power is controlled by varying the length of the power pulse and the duration of the open circuit. For example, by lengthening the power pulse portion and decreasing the time of the open circuit portion increases the average power delivered to the motor.
The power pulse has a long enough duration for the motor to respond. However, the mechanical system such as the liftgate and motor will also respond during the duration of the second portion when the power is off. Thus in a lift gate for example, the liftgate angular velocity will slow down and may even reverse drive the motor due to the influence of gravity on the lift gate during the time of the open circuit. Thus one solution is to utilize a non-reverse drivable gear motor, such as a worm gear coupling which acts as a mechanical breaking device that prevents the gate from falling in between pulses of power. However, such coupling also prevents manual override of such lift gates and power doors and does not allow a person to manually speed up the motion if so desired.
The present systems also do not slow down the motor if it is running too fast thus not providing full motor speed control.
Alternative systems with overriding clutches and reverse drivable motor allow for the manual override but do not provide for a braking feature.
Dynamic braking systems are also known which provide pulsed braking power to the electric motor. Such dynamic braking systems cycles also are similarly divided into two portions, an open portion where no braking is applied and a short circuit portion where braking occurs. The pulses vary between an open circuit and a short circuit which controls the desired dynamic braking effect. The average braking power is controlled by varying the duration of the braking or short circuit and the duration of the open circuit. The pulsing between open and braking pulses thus dynamically control the amount of braking in such motors in much the same way as the PWM cycle during the powering of the motor. Lengthening the time of the short circuit portion and shortening the time of the open portion increases the dynamic braking of the motor. Dynamic braking is often applied during the closing cycle of the lift gate or during the period of time when the gas struts take over during the opening cycle of a lift gate.
What is needed is a dynamic damping system which cycles between power pulses and braking pulses for more desirable control of an electric motor for a power sliding door or power lift gate.